Methods for calefaction densification of a porous structure

ABSTRACT

The invention relates to a film-boiling densification method for a porous structure (mixed gas-liquid method) ( 5 ) consisting in immersing of the porous structure into a liquid precursor, a hydrocarbon, for example, and heating the system in order to deposit the decomposition product of said liquid precursor, for example carbon, into the pores of the porous structure, characterized in that the flow of the liquid precursor entering the porous structure is reduced, for example by means of a filter ( 52 ) made of polytetrafluorethylene surrounding the structure so as to reduce the vaporization phenomenon of the liquid precursor around the porous structure to be densified.

This application is a National Stage Application of InternationalApplication No. PCT/FR01/03355, which was filed on Oct. 29, 2001; andpublished WO 02/36520 A1 on May 10, 2002 in French.

TECHNICAL FIELD

The object of the present invention is a densification method for aporous structure and, more precisely, it relates to an improvement ofsaid method, which makes possible reducing the electrical power consumedand increasing the densification rate as well as improving thehomogeneity of the texture of the product deposited.

It will be recalled that the film-boiling densification method(gas-liquid mixed method) for a porous structure consists in filling thespaces of the porous structure by depositing in same a materialidentical to or different from that comprising the structure, by thermaldecomposition of a liquid precursor.

The invention applies to the densification of a porous material, inparticular felts, textiles, needle-punched and three-directionalpreforms, can be advantageously used because of their high mechanicalresistance, their excellent thermal insulating ability, and their goodresistance to shock and abrasion, for producing thermal shielding, brakedisks and nozzle throats.

STATE OF THE PRIOR ART

The basic technique of the film-boiling densification method or rapiddensification is described in particular in the FR-A-2 516 914 document[1]. This technique consists in immersing the porous structure to bedensified into a liquid precursor comprised of a hydrocarbon and heatingit by induction so as to form, by decomposition of the hydrocarbon,carbon or pyrolytic graphite capable of depositing on the inside of thepores of the structure.

This technique has also been used, as described in FR-A-2 712 884 [2],for densifying a porous structure using a ceramic material such as boronnitride, by using as the precursor a liquid precursor chosen from theborazines such as trichloroborazine, for example.

Using this technique, two different materials can be deposited in thepores of a porous structure by creating a composition gradient of thetwo materials inside the structure as is disclosed in EP-A-0 515 186 [3].

Perfections of the device used for this film-boiling densificationtechnique have also been described in U.S. Pat. No. 5,389,152 [4] and inU.S. Pat. No. 5,547,717 [5].

Documents FR-A-2 760 741 [6] and FR-A-2 760 742 [7] propose using otherliquid precursors such as cyclohexane; these precursors are aromaticcompounds comprised of halogen or alkyl derivatives of benzene andnaphthalene. These documents also proposed maintaining another porousstructure of a thickness of at least equal to 3 mm in contact with thestructure to be densified in order to assure densification of the porousstructure over its entire thickness.

In the methods described in the documents cited above, the materialflows are governed by the film-boiling phenomenon and is carried out ina natural fashion, which results in:

poor energy yield, since 50 to 90% of the heating energy passes intoevaporation without being used in the cracking operation, and

non-optimized densification rates, since the thermal gradients are veryhigh and the densification rate diminishes as the thermal gradientincreases.

SPECIFICATION OF THE INVENTION

The object of the present invention is precisely an improvement of thefilm-boiling densification methods for porous structures, which makespossible reducing the electrical power consumed and increasing thedensification rate as well as improving the homogeneity of the textureof the deposited product in the case where it is carbon.

According to the invention, the film-boiling densification process for aporous structure consists in immersing the porous structure, whetherjoined or not to a susceptor, in a liquid precursor and heating thesystem in order to deposit the decomposition product of said liquidprecursor in the pores of the porous structure, and it is characterizedin that the flow or the liquid precursor entering the porous structureis reduced so as to reduce the vaporization phenomenon of the liquidprecursor around the porous structure to be densified. Heating can be ofthe resistive or the inductive type (with or without a susceptor, bydirect coupling to the preform).

The invention thus consists in controlling the arrival of the liquidprecursor in the porous part to be densified. This can be obtained bydisposing around at least one part of the porous structure a filter madeof a material different from that forming the structure, having apermeability of less than that of the structure to be densified. Forexample, a filter having a permeability of 0.05 to 20 D and a thicknessof 50 μm to 2 mm can be used.

By virtue of the presence of this filter around the porous structure,for a maximal temperature identical in the part there is a reduction ofthe electrical energy and an increase in the densification rate.

The reduction of the electrical energy is principally due to the factthat the vaporization of the precursor, which is a very endothermicphenomenon, absorbing energy and cooling the exterior of the porousstructure, is greatly reduced in terms of flow.

The increase of the densification rate is a consequence of the aforesaidphenomenon. It emerges from the fact that the thermal gradients areweaker in the part, because the amount of heat removed is less and thedeposition rate intersects exponentially with temperature, whichproduces a globally more rapid rate of densification.

Finally, in the case, wherein the method of the invention is used fordensifying a porous structure by carbon deposition, the filter alsoimproves, for the range of temperatures least elevated, homogeneity ofthe carbon deposited. It makes possible elimination of the formation ofmosaic or ex-pitch type carbon at the interior of the strands or mesh offibers of the porous structure, while attenuating or mollifying theperturbations due to boiling of the precursor, or the surge penetrationof the liquid into the porous structure.

Thus, according to the invention, these results are achieved by reducingthe flow of liquid precursor entering into the porous structure which isthe opposite of the object pursued by the document [4], wherein, incontrast, the object is to increase the flow of precursor into thestructure by the creation of liquid waves, rather than limiting them.

Likewise, the effect obtained by virtue of the presence of a filter isquite different from that which is obtained with the felt used indocuments [6] and [7].

In these documents, the felt acts as an extension of the porousstructure and is partially densified at the start of the process. Thus,densification of the porous structure over its entire thickness isassured.

According to the invention, the filter does not act as an extension ofthe porous structure, but as a limiter of the entry flow of liquidprecursor into the porous structure.

The filter used in the invention can be made of various materials underthe condition that the material used is inert with respect to the liquidprecursor used and that it withstands the boiling temperature of theliquid precursor.

This filter can be made of mineral fibers, organic fibers or of glassfibers (for example in the form of one or a plurality of layers offabrics).

The material used for the filter is also chosen as a function of thenature of the liquid precursor used, so that it has a suitablewettability by the liquid precursor.

According to the invention, a filter comprised of apolytetrafluorethylene fabric can be used advantageously.

The thickness of the filter is also chosen as a function of the natureof the liquid precursor used for obtaining the flow reduction of theappropriate precursor. The choice of the thickness also allowsmodulating the power and rate of deposition in the porous structure.However, beyond a certain filter thickness, densification results inincreasing porosity, which generally impairs, for the most currentapplications, the quality of the densified material.

According to the invention, the same reduction in liquid precursor floweffect introduced into the part could be obtained, by depositing aroundit, in the reactor, a diffusion barrier comprised of fittings (glassbeads, Raschig rings).

The method of the invention can be implemented for depositing variousproducts, in particular carbon or ceramic compounds or mixtures thereof.

In the case, wherein the porous structure is densified by carbondeposit, the liquid precursor can be a liquid hydrocarbon chosen fromalkanes, cycloalkanes, alkenes, aromatic hydrocarbons and theirderivatives.

The cycloalkane, in particular, can be cyclohexane.

The aromatic hydrocarbons can be chosen from benzene, naphthalene andtheir halogenated or alkylated derivatives.

In the case of deposition of a ceramic compound, the liquid precursorcan be chosen from borazines, alcoholates, silanes and theirderivatives.

According to the invention, another porous structure of a thickness ofat least equal to 3 mm, such as a felt having a thickness of 3 to 10 mm,can be intercalated between the porous structure to be densified and thefilter.

In order to implement the method of the invention, the system can beheated by induction, by Joule effect or by direct coupling or by acombination of these means.

When the heating is an inductive heating implemented with a susceptor ora resistive heating implemented using a resistor, an expansion joint ispreferably disposed between the porous structure to be densified and thesusceptor or the resistor.

Such a joint generally has a thickness of at least 1 mm, advantageouslyat least 3 mm. It can be made of paper or graphite.

The porous structures capable of being densified by the method accordingto the invention, can be in particular textiles or felts of carbon orgraphite.

Generally, densification is carried out on such porous structures bydepositing carbon, starting with the aforementioned liquid precursors.

Other features and advantages of the invention will be come obvious whenreading the following description, the illustrative non-limitingexemplary embodiment, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a densification device thatcan be utilized in the invention

FIG. 2 represents the porous structure to be densified in horizontalcross-section

FIG. 3 represents the temperature profiles in the porous structures inthe form of disks subjected to densification in the presence of a filter

FIG. 4 represents the temperature profiles in the porous structures,identical to those used for FIG. 3, but densified in the absence of afilter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 represents a densification device using induction heating with asusceptor, which can be used for implementing the method of theinvention.

This device comprises a column comprised of three parts: a reactor 1, adroplet separator 2 or aerosol trap and a heat condenser or exchanger 3.The reactor section is equipped with a confinement enclosure or glovebox 4, sweep internally by a neutral gas current. This assures thesafety of the operator in the event of rupture of the reactor and makesit possible to prevent inflammation or explosion of the reaction gas aswell as inhalation of the product by the operator.

The system comprising the porous structure to be densified 5 and thesusceptor 7 is placed on a support 6, which can be rotary or not. Thesupport 6 is mounted in the bottom part of the reactor 1 and passes theclosure lid 8 across which it can slide.

The induction coils 10, which can be arranged inside of the reactor, aresupplied with high-frequency power by a generator 11. The temperaturesof the porous structure and of the susceptor are measured bythermocouples or thermosensors 12, connected to a programmer 13, makingit possible to regulate the power of the generator 11, in order tocontrol the deposit temperature.

Furthermore, the reactor comprises a conduit 14 that enablesintroduction of the precursor continuously to the inside of the reactor1 and a conduit 15 that enables continuous filtering of the reactorcontents, in order to eliminate tars and suspensions generated in it;these two conduits are each equipped with a circulation pump 16 and 17.The reactor is also equipped with a nitrogen conduit 18 or other neutralgas used to expunge the air contained in the reactor 1 at the start ofthe process and, finally, an opening equipped with a valve 19 disposedin the bottom part of the reactor and allowing evacuation of same.

The droplet separator 2, arranged above the reactor 1, acts to eliminatethe mist created in the reactor 1. The condenser or heat exchanger 3,arranged above the droplet separator 2, comprises a serpentine 20 forcirculation of cooling liquid (generally water).

The heat exchanger makes it possible, by cooling the vapors of theprecursor and by condensing them, to send them back to the reactor 1.

The reaction gases are extracted under negative pressure by means of apressure regulating valve 21 and passed through a conduit to a gastreatment installation 22 (neutralization in the event of utilization ofhalogen compounds). The pressure regulating valve 21 is controlled by aregulator 23 connected to a pressostat 24. The gas flow from cracking ismeasured by a flow meter 25. The line 26 enables analysis of the gas.

The installation comprises also a second pressostat 27, in order toidentify the pressure inside the reactor as well as two conduitsequipped with safety release valves 28 and 29, in order to avoid anoverpressure in the column (calibrated at 0.2 MPa). An explosimeter 30is also positioned in proximity to the installation for detecting anycracking gas leakage to the outside of same.

The valves 32, 33, 34 and 35 make possible the use of the pump 16,either for sending precursor P during the implementation of the methodor for circulating solvent S in closed circuit through the conduits 14and 36 for cleaning the installation at the end of experimentation.

FIG. 2 represents a horizontal cross-section of the system formed by theporous structure 5 and the susceptor of FIG. 1 surrounded by payex(graphite paper) 53.

In this figure, it can be seen that the porous structure 5 is disposedaround the susceptor 7 and that it is surrounded by a felt 51 and thefilter 52 according to the invention.

It will be noted that in this figure, the thickness of the filter hasbeen exaggerated with respect to that of the felt, since the filter hasa substantially lesser thickness.

In the following, an mode of implementation of the method of theinvention in this device is described.

The system comprised of the susceptor 7, the porous structure 5, thefelt 51 and the filter 52 are arranged on the support 6 on the outsideof the reactor 1. The thermosensors 12 are positioned, then the reactor1 and the confinement chamber 4 are purged using an inert gas in orderto sweep out any oxygen that may be present. Then the reactor is filledwith a precursor, cyclohexane for example.

After having put the cooling circuit 20 and the gas treatmentinstallation 22 into operation, and powering up the filtration pump 17,the generator 11, the temperature programmer 13, the pressure regulator23, temperature increase of the system is started. The pressure is setto 0.12 MPa with the aid of the pressure regulator 23.

Once the precursor starts to boil, inert gas purging of the reactor issuppressed. When the cracking temperature (around 800° C. forcyclohexane) is reached, the precursor vapor decomposes in the porousstructure, which results in deposition of carbon on the inside of thepores of the substrate.

More precisely, cracking is done at the level of the hottest walls ofthe porous structure. When the porous structure is mounted on asusceptor, the densification front propagates from the face of theporous structure in contact with the susceptor towards the exteriorwall. When there is no susceptor, the densification front progressesfrom the interior of the porous structure towards its exterior wallsplaced in contact with the liquid precursor.

The rate of advance of the densification front can vary by severaltenths of a mm/h to cm/hr as a function of the maximal temperature ofthe porous structure and its nature (type of porosity). The temperatureis controlled by the programmer 13 connected to the thermocouple 12placed in the susceptor 7 (or at the center of the porous structure 10,when there is no susceptor). The measurement by the flow meter 25 of thereaction gas flows and the identification of their composition enablescalculation of the rate of advance of the densification front.

Continuous addition of precursor is done in order to conserve a constantquantity of precursor in the reactor.

The reaction gas mixture, non-cracked vapor and aerosols produced in thereactor is evacuated in the upper part of same. The aerosols and thevapors are condensed in the droplet separator 2 and the exchanger 3; thereaction gases are extracted in the upper part of the installation andeventually neutralized in the gas treatment installation 22. At the endof densification, the measured gas flow drops significantly. Thetemperature is then reduced progressively until reaching ambienttemperature.

The system thus obtained is then recuperated and subjected to thermaltreatment at approximately 500° C. in the furnace under vacuum, in orderto remove the residual precursor impregnating the remaining porosities.The susceptor and the filter are separated from the system and theexternal part of the structure not densified (the added felt, if therewas one) is machined.

In the case of a carbon deposit, the densified structures obtained arehomogeneous, of a density greater than or equal to 1.7 and have, asdemonstrated by optical microscopic characterization in polarized light,a coarse laminar structure. This structure is most interesting, becauseit allows obtaining, by high temperature (2,400° C.) thermal treatment,a crystalline structure approximating that of graphite.

In the following, two examples of carbon densification of parts aredescribed, by using cyclohexane as the liquid precursor and by givingthe results at the time of densification with or without a filter.

EXAMPLE 1

The reactor used has an inside diameter of 200 mm, an height of 300 mm.The inductor that is arranged inside the reactor, has a height of 150 mmand is comprised of six turns having inside and outside diameters havingvalues of 175 mm and 195 mm, respectively.

The susceptor used has a diameter-of 80 mm and a height of 100 mm. It isentirely covered using three pieces of carbon felt (density 0.40 to0.45) to be densified:

-   -   a hollow cylinder having inside and outside diameters,        respectively, of 80 and 120 mm and a height of 100 mm, covering        its lateral surface;    -   two disks having a diameter of 120 mm and a thickness of 20 mm        covering the top and bottom parts of the two flat surfaces.

The system is covered with a filter formed of two layers ofpolytetrafluorethlylene GORE-TEX®, having the following characteristics:

-   -   thickness of one layer: 0.2 mm;    -   filtration: allows passage only of particles of a diameter less        than 7.5 μm;    -   permeability: 1 Darcy (or 1 μm²);    -   thermal conductivity: 0.045 W/m.K at 50° C.; 0.054 W/m.K at 100°        C.

The total thickness of the filter is 0.4 mm.

Pressure is set at 0.1 MPa. The temperature increase is done at a rateof 500° C./h up to 1,100° C. Power is adjusted over time so as to keepthe cracking gas flow almost constant. After approximately 7 h ofdensification, the temperature is reduced at a rate of 800° C./h.

The results are as follows:

-   -   the densification rate is 3 mm/h;    -   energy consumption is 65 kWh/kg of carbon deposited;    -   the carbon is deposited homogeneously and has a coarse laminar        type structure;

By comparison, without GORE-TEX® filter under the same conditions,densification takes about 10 h:

-   -   the densification rate is 1.9 mm/h;    -   energy consumption is 120 KWh/kg;    -   deposit is not homogeneous, since in the fiber strands, in part        externally, there are mosaic type carbon or ex-pitch carbon        deposits.

EXAMPLE 2

This example relates to the densification of small pieces of carbon.Heating is done resistively.

The heating element is a bar of graphite 3 mm in diameter. It issurrounded by the sample to be densified, which is a tube of carbon felt(density 0.1) 2 cm in diameter and 3 cm in height. The system isenveloped in a filter formed of two layers of GORE-TEX® tissue, as inExample 1.

The pressure is set at 0.1 MPa. The temperature increase is done at arate of 1,000° C./h up to 1,100° C. The temperature is held at 1,100° C.for 30 minutes, then lowered at a rate of 1,000° C./h.

The results are as follows:

-   -   the rate of densification is 4 mm/h;    -   the energy consumed is 110 kWh/kg of carbon deposited;    -   the density is 1.8.

By comparison, if there were no GORE-TEX® filter, under the sameconditions:

-   -   the rate of densification is 0.6 mm/h;    -   the energy consumed is 1,400 kWh/kg of carbon deposited;    -   the density is 1.8.

FIGS. 3 and 4 represent the temperature profiles in the 20 mm carbondisks densified in Example 1.

In FIG. 3, the curve Ts represents the temperature variation (in ° C.)of the susceptor as a function of time (in hours and in minutes). Thefollowing curves indicate the evolution of the temperature (in ° C.) asa function of time (in hours and in minutes) for the regions of the disksituated at the distances indicated (in mm). Each distance correspondsto the distance between the place on the part, where the temperature ismeasured and the susceptor.

FIG. 4 represents the temperature profiles obtained under the sameconditions on 20 mm thick disks, in the absence of a GORE-TEX® filter.

By comparing these two figures, it is noted that the most rapid increasein FIG. 3, whose that the thermal gradients are lower. As the rates ofdeposit on the fibers and the carbon yields converge with temperature,the highest temperature values in the part, that is a lower gradient inthe case wherein a GORE-TEX® filter is used, result in more rapiddensification, lower energy consumption and a higher yield in carbon.

It is thus confirmed that the use of a filter according to the inventioncontrols the liquid/gas interface and simply changes the vapor flux onthe inside of the parts, which makes possible significant gains relativeto the densification rate and the electrical energy required fordensification, important economic factors in manufacturing the parts.

DOCUMENTS CITED

-   [1] FR-A 2 516 914-   [2] FR-A-2 712 884-   [3] EP-A-0 515 186-   [4] U.S. Pat. No. 5,389,152-   [5] U.S. Pat. No. 5,547,717-   [6] FR-A-2 760 741-   [7] FR-A-2 760 743

1. A method for film-boiling densifying a porous structure, comprising:a) depositing, around at least one part of the porous structure, afilter made of a material different from that forming the porousstructure said material being a textile of mineral or organic fibers andsaid filter having a permeability lower than that of the porousstructure; b) immersing the porous structure with the so-depositedfilter in a liquid precursor which decomposes under heat; c) heating theporous structure with the so-deposited filter in the liquid precursor todecompose the liquid precursor and to deposit the decomposition productof said liquid precursor into the pores of the porous structure, untilthe porous structure is densified, wherein a flow rate of the liquidprecursor into the porous structure in the presence of the so-depositedfilter is less than a flow rate of the liquid precursor into the porousstructure in the absence of the so-deposited filter.
 2. The methodaccording to claim 1, wherein the filter has a permeability of 0.05 to20 Darcy and a thickness of 50 μm to 2 mm.
 3. The method according toclaim 1, wherein the filter is a polytetrafluorethylene textile.
 4. Themethod according to claim 3, wherein the filter has a permeability of 1Darcy and a thickness of 0.2 mm.
 5. The method according to claim 1,wherein another porous structure of a thickness at least equal to 3 mmis intercalated between the porous structure to be densified and thefilter.
 6. The method according to claim 1, wherein the liquid precursoris a liquid hydrocarbon chosen from alkanes, cycloalkanes, alkenes,aromatic hydrocarbons and their derivatives.
 7. The method according toclaim 6, wherein the liquid precursor is cyclohexane.
 8. The methodaccording to claim 6, wherein the liquid precursor is an aromatichydrocarbon chosen from benzene, naphthalene and their halogenated oralkylated derivatives.
 9. The method according to claim 1, wherein theliquid precursor is a precursor of a ceramic compound chosen fromborazines, alcoholates, silanes and their derivatives.
 10. The methodaccording to claim 1, wherein the heating of the system is a heating byinduction using a susceptor and/or a resistive heating by Joule effectand/or a heating by induction by direct coupling.
 11. The methodaccording to claim 10, wherein the heating is an inductive heatingimplemented using a susceptor or a resistive heating implemented using aresistor; an expansion joint being disposed between said porousstructure to be densified and said susceptor or said resistor.
 12. Themethod according to any one of claim 1, wherein the porous structure ismade of carbon and the liquid precursor is cyclohexane.